1. Field of the Invention
The present invention relates to a two-dimensional device array in which many active or passive devices such as light emitting devices and displaying devices are arranged in two dimensions, to a two-dimensional surface light emitting laser array in which many laser elements are arranged in two dimensions on a semiconductor substrate, to a method for driving such a two-dimensional device array and such a two-dimensional surface light emitting laser array, and to an image forming apparatus utilizing such a two-dimensional surface light emitting laser array as a light source for exposing process.
2. Description of the Related Art
A two-dimensional device array has been developed as an apparatus for emitting light, displaying an image and detecting a position within the predetermined surface by arranging in two dimensions a large number of active or passive devices such as light emitting devices and displaying devices, and this two-dimensional device array has now been put into the practical use.
However, in the case of individually driving the devices of such a two-dimensional device array, a number of wirings as many as the number of devices are required, resulting in a problem that the number of wirings increases in proportion to the number of devices.
For example, in the case of the two-dimensional surface light emitting device array having n.times.m devices arranged two-dimensionally with n devices in the row direction and m devices in the column direction to emit light when a voltage higher than the predetermined threshold voltage is applied across the anodes and cathodes of the devices, n.times.m wirings are required for the anodes in order to individually drive the devices although the cathodes can be integrally formed as a common electrode and grounded.
In view of reducing the number of wirings, it is known that many devices can be connected to the wirings in the vertical and horizontal directions by the matrix wiring method. For example, it is indicated in the Electronics Letters Vol. 27, 1991, pp. 437-438, U.S. Pat. No. 5,031,187 and Photonics Technology Letters Vol. 8, 1994, pp. 913-917 that many lasers are matrix-wired in the vertical and horizontal directions in a two-dimensional surface light emitting laser array in which many laser elements are arranged two-dimensionally on a semiconductor substrate.
FIG. 27 shows an example of the vertical and horizontal matrix wiring of the related art for a two-dimensional surface light emitting laser array in which laser elements 1 are arranged in two dimensions, namely n laser elements are arranged in the row direction while m laser elements in the column direction, the anodes of m laser elements arranged in the column direction are connected to an anode wiring 2 forming the column wirings, the cathodes of n laser elements in the row direction are connected to a cathode wiring 3 forming the row wirings, an anode pad 4 is formed at an end of the anode wiring 2, and a cathode pad 5 is formed at an end of the cathode wiring 3.
In practise, the anode wiring 2 and the anodes of m laser elements arranged in the column direction are integrally formed and the cathode wiring 3 and the cathodes of n laser elements arranged in the row direction are also integrally formed. In the example of FIG. 27, it is also possible that the row wiring is used as the anode wiring and the column wiring as the cathode wiring.
When a voltage higher than the predetermined threshold voltage is applied across one of the n column wirings, and one of the m row wirings in the two-dimensional surface light emitting laser array of the vertical and horizontal matrix wiring, a laser located at the intersecting point of these selected wirings emits light. All the laser elements in the array can be addressed sequentially to make surface light emission by driving the array with the predetermined scanning pattern. According to the vertical and horizontal matrix wiring, the number of wirings for the two-dimensional surface light emitting laser array is reduced to (m+n) from n.times.m wirings for the individually addressed array.
However, in the two-dimensional surface light emitting laser array of the vertical and horizontal matrix wiring as shown in FIG. 27, electrical resistance exists in the row and column wirings. There also exist various kinds of electrostatic capacitances exist, for example, the laser 1 has an electrostatic capacitance in itself and a stray capacitance is present at the intersecting points of the row and column wirings. When the array becomes large in size, namely when the wirings become longer and the number of devices connected to one wire increases, the wiring resistance and electrostatic capacitance become larger.
Therefore, when the two-dimensional surface light emitting laser array of the vertical and horizontal matrix wiring as shown in FIG. 27 is made large in the row direction direction and thereby the number of the devices n in the row direction is increased, it becomes difficult to drive the array at a high speed and at a low power, because the drive pulse is delayed due to the wiring resistance and electrostatic capacitance and because unwanted charging or discharging current flows into the wiring resistance and electrostatic capacitance to cause unwanted power consumption and heat generation.
Moreover, in the two-dimensional surface light emitting laser array of the vertical and horizontal matrix wiring as shown in FIG. 27, the number of laser elements which may be driven simultaneously is limited to n laser elements connected to one row wiring or m laser elements connected to one column wiring.
Therefore, when the n or m laser elements are driven simultaneously to drive the array at a high speed, and when the array is increased in size in the row or column direction to increase the number of devices n or m in the row or column direction, considerable heat is generated in the row or column wiring which is connected to the n or m laser elements driven simultaneously and thereby the laser characteristics may be deteriorated. On the other hand, if the n laser elements connected to one row wiring or the m lasers connected to one column wiring are divided in parts and each part is sequentially driven to suppress generation of heat, the period required for scanning and driving all the laser elements in the array becomes longer.
One or both of the row wirings and column wirings can be divided into two sections in the two-dimensional surface light emitting laser array of the vertical and horizontal matrix wiring.
For example, the row wirings are divided into two sections and the laser array is also divided into two sections in the point of view of wiring. According to this structure, the wiring resistance and electrostatic capacitance of the row wirings can be reduced to 1/2 in comparison with the case where the dividing is not performed as shown in FIG. 27 and a total of 2m laser elements connected to two column wirings or a total of n laser elements connected to two row wirings may be driven simultaneously.
In this case, the total number of the row wirings and column wirings is increased to (2m+n) and the row wiringa divided into two sections may be extracted from two sides opposed to the row direction of the array.
Otherwise, both the row wirings and column wirings are divided into two sections to divide the laser array into four sections from the viewpoint of wiring. According to this structure, the wiring resistance and electrostatic capacitance of the row wirings and column wirings can be reduced to 1/2 in comparison with the case where dividing is not performed as shown in FIG. 27 and a total of 2m lasers connected to four column wirings or a total of 2n lasers connected to four row wirings may be driven simultaneously.
In this case, the total number of the row wirings and column wirings is increased to 2 (m+n) and these wirings may be extracted from the four sides of the array.
However, even when the row wirings or column wirings are divided into two sections, n/2 or m/2 lasers are connected to a row or column wiring, respectively, resulting in the problem that the wiring resistance and electrostatic capacitance become large in proportion to the size of array.
For example, a two-dimensional surface light emitting laser array may be used in a laser beam printer as a light source of exposing process, in which the row direction of the array is considered to correspond to the main scanning direction and the column direction to correspond to the sub-scanning direction. The array is driven with the predetermined scanning pattern based on the image data, the photosensitive material drum is irradiated with the laser beams from the array via an optical system to form an electrostatic latent image on the photosensitive material drum, and the electrostatic latent image is developed to a toner image and then transferred on a sheet of recording paper. In this case, several hundreds of laser elements are required as the number of laser elements n in the row direction of the two-dimensional surface light emitting laser array.
Therefore, even when the row wirings are divided into two sections, the wiring resistance and electrostatic capacitance of each row wiring become considerably large. As a result, when the lasers are driven electrically, the response to the drive pulse is delayed due to the large wiring resistance and electrostatic capacitance. Accordingly, it is now difficult to drive the array at a high speed, and at the same time, unwanted power consumption and generation of heat are caused by unwanted charging/discharging current flowing into the large wiring resistance and electrostatic capacitance.
When the row wirings are divided into many sections, the wiring resistance and electrostatic capacitance of each row wiring may be reduced, but in this case, the total number of the row wirings increases and thereby the advantage of the matrix wiring is lost and it is now difficult to extract the row wirings out of the array except for both end portions thereof in the row direction.
This problem may also be considered for a two-dimensional device array which executes emission of light, display of image or detection of position within a constant display surface by arranging two-dimensionally a large number of active or passive devices such as light emitting devices and displaying devices other than semiconductor lasers.
Namely, in the ordinary two-dimensional device array in which many devices arranged in two dimensions are matrix-wired in the vertical and horizontal directions (row and column directions), electrical resistance exists in the row wirings and column wirings. There also exist various electrostatic capacitances. A device has electrostatic capacitance in itself and a atray capacitance is present at the intersecting points of the row and column wirings. When the array becomes longer in one direction, namely when the number of devices connected to one wiring in one direction increases, the wiring resistance and electrostatic capacitance of the wiring in one direction increase. Thereby, when the devices are driven electrically, various disadvantages such as delay of operation in the devices, increase of power consumption, and increase of crosstalk between the devices may be caused by the large wiring resistance and electrostatic capacitance.